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I bought a 2500 watt engine/generator assembly, which i used for a year or so. Then two things became apparent: 1) i needed the engine to exhaust outside 2) i'd had enough of the gas tank leaking gasoline all over the ignition coil. And it needed electric starting, but it had no oem starter and no ring gear on the flywheel.
So, i tore down almost everything outside the block and cylinder head. Popped the flywheel off and roll-pinned a ring gear from a recycled flywheel. I had taken the gear to a cycle repair shop, and purchased a new 12vdc starter to fit it. Naturally, i had to make adapter to fit the starter mount to the available boltholes on the crankcase, and a sheet steel shroud to cover the starter's clutch and drive gear. Then, the gas tank went to the trash, i added a frame and put a 2 gallon tank on it, and routed the new gas line, with a replaceable filter, well away from anything hot or electrical. Next came the battery rack, starter relay, and testing. I made up a flange to accept 3/4 iron pipe, bolted it up where the thin sheet metal "muffler" was, installed a decent muffler, made up and installed "tin" heat shields tween the engine, exhaust system, and carburator.
Everything worked *great* until it didn't.
The engine rocked with vibration, as single cylinder 4-strokes on rubber mounts do, and the direction of this rocking was literally pulling the engine away from the new exhaust pipe, pulling the threads right out of the bolt holes in the cylinder head. Ouch. And the entire exhaust system opened up while it was running during a power outage, outside in it's carpetted sound proof box, blowing hot exhaust right at the carpet. Melted the carpet.
Don't modify the exhaust on an engine unless you're doing something about the vibration.
Here's pics of the exhaust i am not using any more. I resurfaced (ground flat again) the flange after brazing to it. The short pipe goes thru the flange and fits a shallow depression in the head opening. I had brazed both sides.
Look where the oem fuel line is here, close to the exhaust port, laying on the edge of fan shroud steet steel, and how close the carb is. After this pic and the new tank, i reassembled all the oem sheet steel around the flywheel and added additional heat shields.